DropDownList “has a SelectedValue which is invalid

问题:

I have a ASP.NET DropDownList, which I data-bind to a list of objects, and then it throws an exception. This error is driving me nuts, because the message is wrong, and the value clearly does exist in the list of items, as shown by the screenshot:

Here's the relevant ASP.NET markup:

<asp:DropDownList ID="_ddlRequests" runat="server" AppendDataBoundItems="true" DataTextField="RequestNumber" DataValueField="RequestNumber">
    <asp:ListItem Text="" Value="" />
</asp:DropDownList>

回答1:

It may be worth first databinding and then adding your blank value. Something along the lines of:

_ddlRequest.Datasource = request;
_ddlRequest.DataBind();
_ddlRequest.Insert(0, new ListItem("", ""));

The only reason I suggest this is that I came across this before and after banging my head in frustration for a while just settled on doing it this way instead.

回答2:

You can use below code to clear data on DropDownList

_ddlRequest.Items.Clear()